Method for manufacturing solid current conducting elements



c. E. RANSLEY 3,314,876 ACTURING SOLID CURRENT CONDUCTING ELEMENTS 16, 1960 3 Sheets-Sheet 1 A rl 18, 1967 METHOD FOR MANUF Original Filed Dec.

April 18, 1967 c. E. RANSLEY 3,314,876 METHOD FOR MANUFACTURING SOLID CURRENT CONDUCTING Original Filed Dec. 16, 1960 ELEMENTS 3 Sheets-Sheet z' INVENTOR CHARLES ERIC RANSLEY BY ATTORNE ELEMENTS 3 Sheets-Sheet 3 ATTORNEY Aprl 18, 1967 c. E. RANSLEY METHOD FOR MANUFACTURING SOLID CURRENT ONDUCTING Original Filed Dec. 16. 1960 United States Patent Ofitce This application is a division of Ser. No. 76,265, filed Dec. 16, 1960, now abandoned, which application Ser. No. 76,265 is a continuation-in-part of application Ser. No. 764,725 filed Oct. 1, 1958, and now U.S. Patent 3,215,615, issued Nov. 2, 1965.

This nvention relates to improvements in solid current for the production of aluminum or a three-layer refining element constitutng the cathode of a reduc the borides of titanum, zirconium, tantalum and niobium and particular reference is made to the excellent properties of titanium diboride TiB In particular, it has a much lower electrical resistivity than titanium carbide, is more resistant to oxidation in the Although the borides of the elements referred to are significantly superior to the carbdes thereof for the purposes in View they Were, until hitherto, more expensive to produce than the carbides. In the specification of British Patent No. 826,635 I have described and claimed composed of a mixture of titanum carbide and titanium boride. Such a mixture has various advantages which could not be anticipated from determinecl separately and special reference is made to the surprising reduction in the solubility of ttanium carbide in aluminum at high temperatures (e.g., at 970 C.) which can be achieved by small percentage additions of titanium boride, additions of 5 to 25% by weight of ttanium boride being particularly mentioned.

It was found that the oxygen content of the carbides of titaniurn, zirconium, tantalum and niobium is an mportant factor in the solubility of the compounds in molten aluminum. In my co-pending application Ser. No. 764,725 filed Oct. 1, 1958, now Patent No. 3,21S,615, I have described and claimed a solid current conducting element for use in an electrolytic cell for the production titanium, gen content of said materials is less than 1% by Weight.

The expressions largely composed of and "consisting essentially of as used hereinafter in the specification and the claims, mean that the current conductmg element, or

such desirable characteristcs, for example, small propor- In order' for the element to consist essentially or be largely composed of at least one of the carbides and/or borides as described above the oxygen impurity content should be limited as the portion thereof adapted to contact molten aluminum during use, desrably, but not essentially contains at least -by weight of at least one of the carbides and/or borides referr'ed to. i

Although the borides of the Chemical elements referred disintegrate within the cell. Micrographic eXamination of elements or -bars after service showed that in poor materials exhbiting this deficiency, a characteristic intergranular penetration of alumnum had occurred which pores, and did These efects could be alumnum as well as under elec- This quality factor could in fact be dihedral angle" C. S. Smith in the American Institute of Mining produced in molten pure trolytic conditions.

30) disintegration and the material was Patented Apr. 18, 1967 In order to indicate how good and poor materials may be distinguished by appropriate chemical analysis and to give an understanding of the complex Constitution of these materials, it is desirable to indicate the processcs by which they may be manufactured.

Continually improving techniques have materially reduced the cost of preparing titanium diboride, TiB and various processes are available for its preparation. The most important ones are considered to be the carbothermic process, electrolytic separation and direct reaction between the Chemical elements. The carbothermic process may involve any one of several reactions, one of which involves reacting TiO (anatase or rutile), B O and carbon according to the following equation:

An alternative reaction is one in which the boron is supplied in the form of boron carbide (nominally B C):

Elemental boron may also be used in -accordance with the equation:

follows:

TiC+TiO +B C- 2TiB +2CO (4 Reactions (2), (3) and (4) are obviously variants of the basic reaction given in Equation 1.

In addition to carbothermic .processes, the most direct way of prcparing the borides is by reaction of the elements:

Ti+2BTiB (5) In this case again, however, the main objection is the difficulty of preparing elemental boron of the necessary purity at an economical price.

Various electrolytic processes have also been proposed for the preparation of the borides. These do not normally offer any technical or economic advantage over the carbothermic or direct reaction processes; however, even for this product, the considerations to be detailed below are still applicable.

It is extremely diflicult in practice to achieve an exact stoichiometric balance in a carbothermic, or any other reaction for the preparation of TiB as is implicit, for example, in Equation 2 above. There are technical difiiculties, for example, in providing exactly the correct amounts of boron and carbon; this is because somewhat variable losses can occur during the reaction, accidental contamination may take place, and the atmosphere in which the process is carried out may also have an effect on the composition of the final product. A further possibility is that the reaction may not have proceeded as far to completion as the make-up of the reaction mass would allow under appropriate conditions.

The product obtained from such reactions will thus contain, to a greater or lesser degree, Components other than the simple compound TlBz. Thus, if the reaction mixture contains an inadequate supply of boron or boron compounds to combine with all the titanium present, and carbon is present over and above that required for the elimination of oxygen, the product will contain an appreciable amount of titanium carbide, TiC. In fact, it is probably not inaccurate to regard TiC as a primary product of the carbothermic preparation; it tends to be unstable in the presence of boron, however, in conformity with the reaction:

TiC+2BeTiB +C (6) Similarly, if the mixture is deficient in both boron and carbon, the product will be contaminated with oxygen in some form or other. The other possible variations in composition which can occur do not need detailed description, but it will be evident that the final product may possibly contain, for example, excess boron carbide and free (or uncombined) carbon. In addition, a number of contaminating elements may be present which are derived from the raw materials or from the atmospheres used; these may include, for example, relatively small quantities of nitrogen, iron, calciu'm, silicon and aluminum.

The final product of usch carbothermic or other processes is normally in the form of a relatively fine powde', which has been subjected to milling or other Operations to render it suitable for further fabrication.

The solid conducting elements referred to are produced by either cold pressing followed by sintering, or by hot-pressing this powder, an appropriate protective atmosphere being used in both these Operations. For example, it may be hot pressed in a graphite die, in a protective atmosphere of hydrogen, and at temperatures of the order of 1800-2100 C. It will be understood that the Chemical composition of the final element thus obtained will be deternined not only by the exact composiion of the powder used, but also by the conditions under which the element is manufactured from it. Precise control of the final composition of the element is essential, however, if the latter is to have a useful economic life in an electrolytic cell.

We have found, unexpectedly, that the elements exhibiting poor behavior do so because of the presence therein of a relatively small percentage of oxygen combined in a certain manner, and it has further been found that the permissible content of oxygen of this type is related to the so-called soluble, or combined, carbon content of the material. The discovery of this interrelation between the oxygen and carbon contents of such elements has enabled us for the first time to control their composition in such a way that no cracking or disintegration occurs in service.

According to the present invention, a solid conductng element for use in the electrolytic production or pttrification of aluminum has `at least `a portion consisting essentially of at least one of the materials titanium diborde and titanium carbide and has an oxygen impurity content, defined as that oxygen present in the element in chemical combination with titanium which is less than O.l% by weight when the combined (or acid-soluble) carbon content is less than 04% by weight, and is less than (0.1+0.04- n)% by weight when the combined carbon content is 11% by weight, and the value of is greater than 0.4.

Preferably the element contains titanium carbide in the proportion of not more than 40% by weight and desirably not less than 2% by weight.

From the analytical point of view, the proportion of acid-soluble carbon present defines the combined carbon content, which is normally ascribable to the formula TiC. In analysis, this figure is usually derived by carrying out a determination of the total carbon content of the material, and a separate determination of the acid-insoluble or filtrable carbon residue remaining after the mass of the sample is dissolved in an appropriate acid or acid mixture; this latter figure thus includes graphitic and elemental carbon, and also carbon in the residual boron carbide. The acid-soluble carbon is then obtained from the diference between these two figures, and this is the value of carbon content which is specified above to which the tolerable oxygen content is related.

The oxygen content of the final element may conveniently be determined by the well-known vacuum fusion method, in which a sample is reduced in a high temperature bath of molten iron or platinum contained in a graphite -crucible, and the consequent evolution of carbon oxide measured. The sample may, for example, comprise a relatively massive chip or cut section of the element. If the element is powdered in order to obtain analysis, spurious oxygen will be introduced n the form of the particles, and also as absorbed water; appropriate correction must therefore `be made in order to obtain the oxygen content which is significant in the present context. Methods are available for this; thus the B O content can be determined by aqueous extraction of the powder, and the water content can be deduced from the hydrogen evolved in the vacuum-fusion measurement.

It is clearly essential to be able to assess the quality of element which will be obtained by hot-pressing or otherwise consolidat ing a given batch of powder into the said element. Since it is desired that the oxygen content, as defined above, of the final product shall be as low as possible, a minimum requirement is that the proportion of reducing agents present shall be adequate to eliminate this oxygen from the powder.

Both carbon and boron are capable of functioning as reducing agents. For reasons which are explained in more detail below, it has been found preferable to adjust the composition of the charge in the carbothermic reaction so that the :powder for pressing and also the final element contains an appreciable percentage, eg., 02% or more, of acid-soluble carbon, or about 1% or more of TiC. Oxygen will he evolved from a powder of this type during hot-pressing or sintering by reaction with the free, or other available, carbon with a loss of carbon monoxide from the system. Oxygen present as titanium oxide (e.g., TiO) is able to -react as follows:

and the carbon required render these conditions such that the Weight ratio O/C is not greater than %4=O.66. However, if boron is available in the powder either as unreacted boron or boron carbide, or even as B O further carbon is made 'available by reaction of this boron with TiC to form TiB and releasing carbon as expressed in Equation 6. Under these conditions the basic carbon requirement is then given by the carbon available from =reaction 6 in percent by weight is 0.56 times the percentage of boron present in the powder other than as TiB A requirement for the control of quality of the powder is thus:

Total O to be reduced I (MI O available for reduction Total O by vacuum fus i91 Free C+0.50 cntent 1.33 other than TB;)

If a mixture is used in *which an excess of boron is intended, the TiC content of the carbothermic powder tends to he low by vi rtue of the reaction shown -in Equation 6, i.e., the carbon is displaced by boron. The undesirable oxygen content can then -be reduced by reaction wth boron; the product obtained, B O is not harmful to quality and since it is quite volatile at high temperatures, some is lost from the element during the hot-pressing or sintering operation. There are some very inconvenent consequences of this type of composit-ional control, however, which render it undesirable. Thus, a high B O content tends to cause -sticking of the powder in the die during hot-pressing so that a high degree of compacton cannot be obtained without adopting special precautons. In addition, it is found that the electrical resistivity of the final element is higher than in materials with an appreciable 'FiC content and that the oxidation resistance is also poorer.

Accordingly, the present invention also provides a powder for use in the manufacture of a solid current conducting element for use 'in an electrolytic cell 'for the production of aluminum which powder consists essentally of at least one of the materials titanium dborde and titanium carbide and wherein the ratio of total oxygen present in the powder n percent by weight to the powder for reduction in 3.

that required for reduction in an amount sufiicient to compensate for this deficiency of boron.

The invention also provides as a 'further feature, a method of producing a solid current conducting element for use in an electrolytic cell for the production of aluminum which comprise-s hot-pressing or cold-pressing and subsequently sintering a powder according to either of the two immediately preceding paragraphs.

It is preferred to hot-press the powder to a density such that the porosty does not exceed 10% by Volume although a greater porosity can be tolerated provided it does not exceed 20% by Volume.

As mentioned above, commercial production of titanium diborde (e.g., using reaction 2 above) automatically introduces a number of contaminating elements of which the principal ones are iron, carbon, nitrogen and oxygen. Traces of other elements such as calcium, silicon and aluminum are also present, usually totallng not more than 05% by weight and these, in such small proportions, do not appear to be significant. -Carhon and oxygen usually derive from the basic material and nitrogen from two main sources (a) original impurty in the starting material and (b) pick-up in grinding and ball-milling Operations. Generally speaking, the iron content should be less than 05% `by weight although a higher content can he tolerated. The nitrogen content should be less than 03% 'by weight for a percentage by weight of combined carbon of up to 04%, not more than 06% by weight at 10% combined carbon (corresponding to 50% by weight of titatnum carbide) and not more than 1% by Weight at 20% combined carbon (corresponding to by weight of titanium carbide) although a higher content can be tolerated.

It is preferred that the free carbon content of the final element should not exceed O.8% by weight.

SiX examples of the manufacture of solid current conducting elements will now be given:

Example I 4200 g. of ball-milled titanium diborde powder prepared by a carbothermic process was taken for fabrication. This powder had a mean cumulative particle size sedimen tation in Water,

deviation of approximately 2.5,u. It had the following Chemical analysis:

(Powder M.18) Weight percent Equivaient compounds Weight percent Acid-semble boron In the above analysis, and in those the so-called acid-soluble boron 7 B O or hydrolysable borides of impurity elements, or both.

The powder was pressed in a 2 in. diameter graphite die in an atmosphere of hydrogen, with an applied pressure which was inereased in appropriate steps from %t ton/sq. in. to a final val-ue of 1 ton/sq. in., the temperature being raised from cold to 2000-2050 C. in about 3 /2-4 hours.

The rod obtained, which was approximately 20 in. long, had the following properties which could be determined by non-destructive testing: Density (overall) (89% theoretical) 4.02 Electrical restitvity, microhm. cm. 19.4

The Chemical analysis of the material, `carried out on a sample of the rod crushed to -90 mesh B.S.S., was as follows:

(Red 13.100/414) Weight Equivalent Weight percent compounds Percent Aed-soluble boren 29.38 TiB 94.3 Titanium 68. 6 Acid-insoluble bo 0.17 Free carbon. 0.29 0 76 TiC 0. 26 0.07 Oxygen as taniun oxide.

The titanium oxide" content was deduced by subtracting the spurious oxygen (associated with water vapor and surface oxidation produced by the powdering process) from the total oxygen. A check figure obtained by measurement of the total oxygen content of a large fragment or chip of bar (which required only a very small correction for spurious oxygen) gave 0.11% oxygen at titanium oxide.

This rod was provided with an aluminum lead at one end in the manner specified in British Patent No. 825,443, and was used as a top-entering lead in an experimental reduction cell as described in my U.S. application Ser. No. 660994, filed May 23, 1957, now Patent No. 3,028,- 324. It carried current for a period of 122 days without any cracking or serious deterioration within the molten flux and metal zone of the cell, and with very little solution (a reduction in diameter of only 0.008 n.).

This was thus representative `of a final fabricated material suitable for cell application.

Example ll 4200 g. of titanium boride powder of similar physical characteristics to that specified in Example I were taken; this batch had the following analysis:

(Powder 211/2) Weight percent E quiv alent conpounds Weight Pereent Acid-semble boren 28. 54 Titaniurn 67. 1 Aeid-insoluble boren 0 07 Free' ear on.

The powder was hot-pressed on a similar schedule to 0 The Chemical analysis of the crushed rod was determined as follows:

The titanium oxide was deduced as in Example -I,

The rod was provided with an aluminum lead as previously described, and was used as a top-entering lead in a reduction cell in the same manner as in Example I. It failed to carry current after a period of immersion of less than 2 days, and when removed for examination showed multiple cracking, mostly of a radial character, which rendered it useless.

Material of this nature was thus clearly quite unsuitable for cell applications.

Example III 2250 g. of titanium boride powder of similar physical characteristics to that specified in Examples I and II were taken; this batch had the following analysis:

(Powder 278) Weight Equivalent Weight percent compounds pereent Aeid-soluble boton..- 29. 32 Ttaniun 1 67. 5 Acid-insoluble boren 0.06 Free" carbon 0.48 Soluble earbon 1.00 Iron 0. 60 Nitrogen 0. 06 Total oxygen 0. 62 99. 64

Density (overall) (89% theoretical) 4.03 Electrical resistivty, microhm. cm 18.0 Transverse rupture strength, tons/ sq. in 15.4

The chemical analysis of the crushed rod was deternined as follows:

(Red 13.100/297) Weight Equivalent Weight pereeut compounds percent Acid-soluble boren 29. TBz.

Titanium 1 Aeid-insoluble boren Free earbou Soluble carbon Iron Nitrogen Oxygen as tit amum oxde."

The "titanium oxide" content was deduced as in Example I.

Half of this bar was subjected to an immersion test in a reduction cell. For this test it was placed in the pool (or pad) of molten aluminum at the bottom of a reduction cell Operating under normal conditions; after immersion for 129 days, it was recovered for examination and found to be quite sound and free from cracks. The

g diameter had decreased by 0.11 in. during this immerson. The rather higher rate of solution than that quoted in Example I a'ose from the conditions of test.

ton conditions, `Was not fully saturated.

Matrial of this composition was thus judged suitable for cell applications.

Example IV 2250 g. of titanum borde powder of similar physical characteristics to that specified in Examples I, II and III were taken; this batch had the following analysis:

This powder was hot-pressed on a similar schedule to that described for Example III.

The rod obtaned was approximately 10 in. long and had the following properties:

Density (overall), (905% theoretical) 4.10

Electrical resistivity, microhm cm. 14.2

Transverse rupture strength, tons/sq. in. 5.7

The Chemical analysis of the crushed rod was determined as follows:

-- (Rod 13.100/241) Equivalent Weight compounds percent Acid-soluble boron Titanium Aeid-insoluble "Free" carbon Soluble earbon Iron N it'ogen Total oxygen The "titanum oxide content was deduced as in Examples I, II and III.

Half of this bar was subjected to an immerson test in a reduction cell in the same manner as described in Example III. This was recovered after 6 days and was found to be badly cracked and virtually disintegrated.

Material of this composition was thns clearly unsuitable for cell applications.

Example V Titanium diboride powder was ball-milled with 10% by weight of titanum carbide powder, and the final milled material was analyzed as follows:

(Powder 365) Weight Eqnivalent Weight percent conipounds pel-cent Aeicl-solubleboron i 26.49 TlBz 'Pitanium 67 4 Acid-nsoluble borom, 0. 44 "Free" earbon 0. 84 Soluble carboL 2. 56 Iron 0 13 N itrogeL, 0. 06 otal oxygen 1. 20

200 g. of the powder were hot-pressed on a similar schedule to that described in Examples III and IV.

10 The rod obtained was approximately 10 in. long and had the following properties:

Density (overall) (952% theoretical) 4.36 Electrical resistivity, microhm. em. 16.6 Tranverse 'upture strength, tons/sq. in. 14.6

The Chemical analysis of the crushed rod was determined as follows:

(Rud B. /3) Weight percent Equivalent compounds Weight pereent Half of this rod was subjected to an immerson test in a reduction cell in the same manner as previously described. It was recovered after 92 days and was found to be quite sound and free from Cracks; the diameter had decreased by 0.08 in.

This material was thus cations.

judged suitable for cell appli- Example VI Titanium diboride powder was ball-milled With 10% by weight of titanum car-hide powder and the: final milled material Was analyzed as follows:

(Powder 363) Weight Equivalent Weight percent compounds percent Acid-solulle boron Titanium Aoid-insoluble b Free an-hon- Soluble carbo on Nitrogen Total oxygen 2000 g. of the powder was hot-pressed on a similar schedule to that described in Examples III, IV and V.

The rod obtained Was approximately 10 in. long and had the following properties:

Density (overall) (973% theoretical) 4.46 Electrical resistivity, microhm. cm. 14.6 Transverse rupture strength, tons/sq. in. 17.7

The Chemical analysis mined as follows:

of the crushed rod was deter- D (Rod 13.90/1) Equ'valent Weight conpounds percent M Acid-solble boron Titanium Aeid-insoluble borou "Free enrbon Soluble carbon Iron Half of this rod was subjected to an immerson test in a reduction cell in the same manner as previously described. It was recovered after 21 days and was found to be badly cracked.

Material of this composition was thus clearly unsuitable for cell applications.

The oxygen content of a solid current conducting element produced as described above, other than that due to B O or oxygen absorbed during powdering of the 1 l element to analyze it, is considered to be mainly present In the example illustrated in FIG. `1 pairs of opposed in the form of oxygen associated with the metal, eg., solid cathodes 1 of plate -form and consisting essentially titanium oxide and it will be observed from the above of at least one of the materials titanium diboride and tiexamples that the elements produced in accordance with tanium carbide are supported by walls 2 at a small angle Examples I, III and V, all of which were suitable for to the vertical one on each side of an appropriately shaped the purposes in View, conform to the requirement that anode 3. The right-hand wall 2 is omitted in this figure. this oxygen content in percent by weight, in relation to The cathodes 1 are immersed in molten electrolyte 4 conthe combined or acid soluble carbon content of the eletaining dissolved alumina and extend at their lower end ment in percent by weight is less than 0.1% when the into the pool of molten aluminum 5 which forms on the combined carbon content is no greater than 0.4% and is floor of the cell. Current leads 6 which also consist esless than -(0.l+0.04 n) percent where is said comsentially of at least one of the materials titanium diboride bined carbon content and is greater than 0.4%. This and titanium carbide extend through the rwall of the cell relationship is due to the fact that when the combined in the pool of molten aluminum 5 and are connected at carbon is greater than O.4%, the oxygen, in the form of their outer ends to the negative pole of the D.C. supply. titanium oxide TiO, can exist in solid solution in the The pool of molten aluminum 5, in this example, forms carbide, which appears as a second phase. part of the current supply system and also acts as a cath- An analysis of the powders used in and of the final ode. The usual crust of solidified flux which forms over elements or rods produced by the above examples is set the cell is indicated at 7. out in the following table. In this table, total carbon The arrangement illustrated in FIG. 2 is very similar available for reduction is calculated on the basis total to that illustrated in FIG. l .and like references are used carbon free carbon+0.56 `(acid-insol B+water-sol B). to denote like parts. In this case, the current leads 6 TABLE I Powder Analysis, perceut by weight Rod Analysis, percent by Weig it Exam le Ratio, o o ualit p Total C l Q y Free C" Acid insol. Water sol. B available Total O O as titani- Residual B for reduction um oxide Free C 0 70 0 23 0.1.5 0 91 1 11 1.2 0.07 0.29 Good o 20 0 07 0.47 o 50 1 53 2. 65 0.63 0.02 Bad. o 48 0 06 0.07 0 54 0 62 1.15 0.08 0.90 Good 0 26 0 04 0. 43 0 52 1. 27 2. 0. 59 0.04 Bad. 0 44 0.12 1 15 1.20 1.05 0.17 0.48 Good 0.85 2.3 0.36 0.03 Bad.

It will be observed from this table that the powders are omitted and replaced by extensions la of the cat hfrom which a good element was produced conform to odes which extend through the crust 7 for connection to the requirernent that the ratio of total oxygen to total the negative pole of the DC. supply. These extensions carbon available for recluction should be less than 1.33. are provided with protective sleeves '3 where they extend It is apparent from our investigations that in a solid through the crust 7, these sleeves being of aluminum or current conducting element consisting essentially of at a refractory material. least one of the materials titanium diboride and titanium The arrangement illustrated in FIG. 3 is similar to that carbide, the quantity of oxygen present in the form of illustrated in FIG. l but in this case, the solid cathodes the oxide of the metal is a determining factor. It would 1 are omitted together with their supporting walls 2 and appear that when this quantity of oxygen is higher than 45 the pool of molten aluminum 5 constitutes the sole catha predetermined value which is a function of the combined ode. or acid-soluble carbon content of the element, the element The arrangement illustrated in FIG. 4 is similar to that is attacked by molten aluminum which penetrates the shown in FIG. 3 but in 'this case the current leads 6 do element and causes it to crack and/ or disintegrate. not extend through the wall of the cell but extend into It will be apparent from the above disclosure that the 'pool of molten aluminum 5 through the crust 7 at the an unsuitable powder can, under appropriate conditions, side of the cell and are provided with a protective sleeve be made suitable by increasing the free carbon content 8. -It is preferred, however, that the current leads 6 thereof and/ or by increasing the boron content thereof should be arranged as indicated in FIG. 5 substantially (other than in the form oli the boride of the metal). centrally of the cell between anodes 3 as this arrangement As higher oxygen contents can be tolerated with increasleaves the sides of the cell free for replenishing the cell ing proportions of the carbide of the metal, a bad powder and the crust 7 is thinner at the center than at the sides. can sometimes be changed to a good powder by increas- Additionally this arrangement lends itself to an arrangeing the amount of titanium carbide present. This is ment of bus-bars (not shown) whereby magnetic effects not a disadvantage as I have found that the very pure can be reduced to a minimum. borides present difllculties in pressing to the final form as FIG. 6 shows an arrangement in which the current they do not densify readily and there is a tendency for leads 6 extend through the floor of the cell. sticking to occur in the graphite diel It is theretore FIG. 7 shows how an existing cell may be modified to preferred that a solid current conducting element consistinclude current leads 6 according to the invention, these ing of at least one of the materials titanium diboride and leads being embedded in the carbon floor 9 of the cell titanium carbide should contain from 2 to 40% by weight which is connected to the negative pole of the DC. supof titanium diboride or titanium carbide and desirably ply by iron bars 10 embedded therein. The current leads not less than 10% by weight. extend up into the pool of molten aluminum 5 through Some examples of electrolytic cells embodying solid any sludge layer which may be formed on :the floor of current conducting elements according to the invention the cell and so avoid the power loss which such a layer will now be briefly described with reference to the acotherwise introduces. companying drawings in which: FIG. 8 shows a three-layer refining cell having the usual FIGS. 1 to 7 are sectional views of reduction cells, lower layer 11 of molten aluminum alloy, intermediate d layer 12 of electrolyte and upper layer 13 of purified alu- FIGS. 8 and 9 are sectional views of three-layer reminum. The lower layer 11 is connected to the positive fining cells. pole of a D.C. supply by current leads a extending i content dened in relation through the wall of the cell and the upper layer 18 is connected to the negative pole of the D.C. supply by current leads 6b extending through the wall of the cell,

the current leads 6a and Gb consisting essentially of at least 1?ne of the materials titanium diboride and titanium caride.

In a three-layer cell illustrated in FIG. 9 the current leads 6b are shown extending into the upper layer 13 from the top of the cell and are provided with protective sheaths 8.

Although the foregoing disclosure relates to titanium carbide and titanium boride and mixtures thereof, it is to be understood that .the carbdes and borides of zirconium, tantalum and niobium are to be included. In connection with these metals, however, due to 'the diferent atomic weights, the percentage weight limitation on the oxygen to titanium carbide and titanium boride and mixtures thereof together with the percentage by weight of the combined acid-soluble carbon content is to be multiplied by a factor equal to the ratio of the atomc weight of titanium to the atomic weight of zirconium, tantalum or niobium as the case may be. Where mixtures of the carbides and/or borides of two or more of the metals are concerned, this factor will be modified in accordance with the relative proportions of the metals present.

It will be obvious that various modi fications and alterations may be made in this invention without departing from the spirit and scope thereof and it is not to be taken as limited except by the appended claims, wherein what is claimed is:

1. A method of producing a current conducting element having high resstance to failure under aluminum producing electrolytic cell Operating conditions, said method compr'sing:

weight of the acid insoluble and water soluble boron materials present in the powder.

4. A method of producing a current conducting element -for use in electrolytic cells for the production of aluminum having reduced tendency to fail under cell operating conditions comprsing sintering at least one powder material having an oxygenand carbon content of a mixture of titanium diboride and titanium carbide, said powder material having a ratio in percent by Weight of total oxygen to total carbon available in the powder material for reduction of oxygen, of less than 1.33.

References Cited by the Examiner UNITED STATES PATENTS 2,813,069 11/ 1957 Raynes et al. 204 -294 2,915,442 12/ 1959 Lewis 204-243 1028324 4/ 1962 Ransley 204-279 JOHN H. MACK, P'mary Examiner. D. R. JORDAN, Assistant Exam'ner. 

1. A METHOD OF PRODUCING A CURRENT CONDUCTING ELEMENT HAVING HIGH RESISTANCE TO FAILURE UNDER ALUMINUM PRODUCING ELECTROLYTIC CELL OPERATING CONDITIONS, SAID METHOD COMPRISING: PRESSING POWDER CONSISTING ESSENTIALLY OF A MIXTURE OF TITANIUM DIBORIDE AND TITANIUM CARBIDE, SAID POWDER ALSO CONTAINING SMALL PROPORTIONS OF OXYGEN AND FREE CARBON; AND MAINTAINING THE PRESENT WEIGHT RATIO O F TOTAL OXYGEN PRESENT IN THE POWDER TO THE TOTAL CARBON AVAILABLE IN THE POWDER FOR REDUCTION OF OXYGEN AT A VALUE OF LESS THAN 1.33. 